We actually use the word "teras" in every day life. It's a literal translation to the word Monster. I don't know why they did change it since its simply called Monster in the Japanese translation

Oh, I see. Indeed, why tamper with the author's intent like that? He already included all the references to Greek mythology that he wanted to. Adding random Greek words on top of that, like they're doing, just comes across as a pointless gimmick.